抄録
The human cytochrome P450 CYP2A6 and CYP2A13 catalyze nicotine metabolisms and mediate activation of tobacco-specific carcinogens including 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL). In this study, we found rhinacanthins A, B, and C isolated from Rhinacanthus nasutus potentially inhibited coumarin 7-hydroxylation mediated by reconstituted purified recombinant CYP2A6 and CYP2A13. Rhinacanthins A-C are mechanism-based inactivators of CYP2A6 and CYP2A13 as they caused concentration, time and NADPH-dependent inhibition. Among the three rhinacanthins, rhinacanthin B possessed highest inhibitory potency against CYP2A13 with apparent KI and kinact of 0.16 µM and 0.1 min-1, respectively, while values of 0.44 µM, and 0.12 min-1 were found against CYP2A6. Rhinacanthin C had least inhibition potency, with apparent KI and kinact of 0.97 µM and 0.07 min-1 for CYP2A6, respectively, and values of 1.68 µM and 0.05 min-1 for CYP2A13. Rhinacanthin A inhibited CYP2A6 and CYP2A13 with apparent KI values of 0.69 and 0.42 µM, respectively and apparent kinact of 0.18 and 0.06 min-1, respectively. The inhibition of both enzymes by rhinacanthins A-C could not be prevented by addition of trapping agents or reversed by dialysis or potassium ferricyanide. These findings concluded that rhinacanthins A-C which are 1,4-naphthoquinone derivatives irreversibly inhibited CYP2A6 and CYP2A13 in a mechanism-based inhibition mode.
